The present invention relates to a surface light-emitting element, a structure for increasing the external light emission efficiency of surface light-emitting elements such as surface light-emitting thyristors, and a self-scanning type light-emitting device using such surface light-emitting elements.
Light-emitting diodes and laser diodes have heretofore been known as typical surface light-emitting elements. Light-emitting diodes rely on a light-emitting phenomenon where light is generated by the recombination of holes and electrons as carriers are injected into the PN or PIN junction formed on a compound semiconductor (GaAs, GaP, GaAlAs, etc.) by forward-biasing the junction. The laser diode, on the other hand, has a construction in which a waveguide is provided inside the light-emitting diode. When a current of a level exceeding a given threshold value is fed, the injected electron-hole pairs increase, causing a reversed distribution that leads to the amplification (gain) of photons due to stimulated emission. With this, laser oscillation takes place as the light generated by parallel reflex mirrors, such as planes of cleavage, is fed back again to the active layer. As a result, laser light is emitted from the end face of the waveguide.
Negative resistance elements having a light-emitting function (light-emitting thyristors, laser thyristors, etc.) are also known as light-emitting elements having a light-emitting mechanism similar to light-emitting diodes and laser diodes. Light-emitting thyristors are of the PNPN structure formed on a compound semiconductor as described above, and have been commercially available as silicon thyristors. These devices are described in detail in pp. 167-169, xe2x80x9cLight-emitting Diodesxe2x80x9d (edited by Masaharu Aoki; Kogyo Chosakai Publishing Co., Ltd.), for example. The basic construction of the negative resistance element having a light-emitting function (hereinafter referred to as light-emitting thyristor is exactly the same as that of the thyristor in that a PNPN structure is formed on an N-type GaAs substrate. Its current-voltage characteristic exhibits exactly the same S-shaped negative resistance characteristic as with the thyristor.
The present applicant has already disclosed self-scanning type light-emitting devices using a surface light-emitting type thyristor (hereinafter referred to as surface light-emitting thyristor) in his patent applications, such as Japanese Laid-Open Patent Publication No. Hei-2(1990)-263668, xe2x80x9cLight-emitting devicexe2x80x9d; Japanese Laid-Open Patent Publication No. Hei-2(1990)-212170, xe2x80x9cLight-emitting element array and method of driving samexe2x80x9d; Japanese Laid-Open Patent Publication No. Hei-3(1991)-55885, xe2x80x9cLight-emitting and light-receiving modulexe2x80x9d; Japanese Laid-Open Patent Publication No. Hei-3(1991)-200364, xe2x80x9cMethod of reading optical signals and switching element array to be used for samexe2x80x9d; Japanese Laid-Open Patent Publication No. Hei-4(1992)-23367, xe2x80x9cLight-emitting devicexe2x80x9d; and Japanese Laid-Open Patent Publication No. Hei-4(1992)-296579, xe2x80x9cMethod of driving light-emitting element arrayxe2x80x9d.
Surface light-emitting elements, such as surface light-emitting diodes and surface light-emitting thyristors, have a problem of poor external light emission efficiency because the light-emitting center is located beneath the electrode for injecting current, making the electrode itself a light shielding layer. This problem will be described in the following, taking the surface light-emitting thyristor as an example.
FIGS. 1A and 1B are a cross-sectional and plan views, respectively, showing a conventional surface light-emitting thyristor of the mesa type PNP structure. Note that these drawings are shown schematically to facilitate the understanding of the construction. This surface light-emitting thyristor comprises an N-type semiconductor layer 24 formed on an N-type semiconductor substrate 1, a P-type semiconductor layer 23, an N-type semiconductor layer 22, a P-type semiconductor layer 21, an anode electrode 40 formed in a such a manner as to make ohmic contact with the P-type semiconductor layer 21, and a gate electrode 41 in such a manner as to make ohmic contact with the N-type semiconductor layer 22. Though not shown in the figure, a cathode electrode is provided on the bottom surface of the substrate 1. On the entire structure shown in FIG. 1A provided is an insulating film (not shown) made of a light-transmitting, insulating material, on which an Al wiring 140 (see FIG. 1B) is provided. In the insulating film provided is a contact hole C for electrically connecting the electrode 40 and the A1 wiring 140. Another contact hole (not shown) is provided in the insulating film on the gate electrode 41 for connecting the electrode to another A1 wiring.
In this surface light-emitting thyristor of the PNPN structure, most of the current fed from the anode electrode 40 flows directly downward, as shown by an arrow in FIG. 1A (indicated by I1.) The light-emitting center of the gate layers 22 and 23 therefore lies beneath the electrode 40. Because of this, light is shielded by the electrode 40 itself and by the A1 wiring 140, lowering the external light emission efficiency.
The amount of light emitted is large in areas near the electrode 40 because of the high density of injected current there, while the corresponding amount is reduced in areas far away from the electrode 40 because the density of injected light becomes smaller. This is one of factors contributing to lowered external light emission efficiency.
Another factor responsible for lowered external light emission efficiency is that part of current injected from the anode electrode 40 flows going round to the gate electrode 41 (indicated by I2). The light emitted by the current I2 cannot be used because it is inclined toward the gate electrode 41. As a result, the amount of light obtained in areas near the anode electrode 40 is reduced.
Japanese Examined Patent Publication No. Hei-5(1993)-25189 discloses a conventional technique of enhancing external light emission efficiency in which the shape of the light-emitting surface of each light-emitting diode in a monolithic light-emitting diode array is made into a U shape by drawing current-feeding wiring to the central part of the light-emitting surface of each light-emitting diode. In this prior-art, however, external light emission efficiency cannot be improved materially because the light-emitting center still lies beneath the electrode.
Japanese Laid-Open Patent Publication No. Hei-4(1992)-259263 discloses a technique of improving light emission efficiency in which the light-emitting region of the active layer in a semiconductor light-emitting element is expanded to a sufficient degree so that the light from the light-emitting region can be extracted without shielding with the electrode on the light extracting side. But the structure and manufacturing method of semiconductor in this technique are considerably complex.
Japanese Laid-Open Patent Publication No. Hei-5(1993)-211345 also discloses a technique of improving external light emission efficiency in a surface light-emitting diode that emits light from a light extracting surface by stacking P-type semiconductors and N-type semiconductors on a substrate, forming a light extracting surface on the topmost part of the stacked semiconductors, and feeding a working current between an upper electrode installed on the light extracting surface and a lower electrode installed on the bottom surface of the substrate, thereby the concentration of impurities at portions other than the portion beneath the upper electrode in a plane parallel with the light extracting surface to form a current control layer that allows the working current to flow easily in the portions other than the portion beneath the upper electrode. In this technique, however, manufacturing process becomes complex.
It is an object of this invention to provide a surface light-emitting element requiring no complicate construction nor complex manufacturing process and having improved external light emission efficiency.
It is another object of this invention to provide a self-scanning type light-emitting device using such surface light-emitting elements.
There are the following three methods of improving external light emission efficiency in surface light-emitting elements, such as surface light-emitting diodes and surface light-emitting thyristors.
(1) The light-emitting center is moved to a position having no light-shielding layer thereabove. To this end, an insulating layer is provided on the electrode region having a light shielding layer thereabove at a portion making contact with the lower semiconductor layer so as to prevent the injected current from flowing from that electrode region.
(2) The peripheral length of the electrode is increased to increase the amount of light emission. With an electrode of the same area, the larger the peripheral length, the more uniformly the current injected from the electrode is distributed and the more uniformly the light is emitted. Thus, the amount of light emission is increased.
(3) When the surface light-emitting element is a surface light-emitting thyristor of a PNPN structure, the construction of the surface light-emitting element should be such that part of the injected current does not flow going round to the gate electrode.
This invention is characterized in that external light emission efficiency is improved, in a surface light-emitting element having a light-emitting layer, an electrode provided on the light-emitting side of the light-emitting layer for injecting current into the light-emitting layer, and a wiring connected to the electrode, by extending the electrode to a region on the light-emitting layer that is not covered by the wiring, and providing an insulating layer under a portion of the electrode covered by the wiring.
The surface light-emitting element of this invention comprises a slender electrode provided on the light-emitting side of the light-emitting layer for injecting current into the light-emitting layer, a first wiring connected to one end of the electrode, a second wiring connected to the other end of the electrode, a first insulating layer provided under an area of the one end of the electrode covered by the first wiring, and a second insulating layer provided under an area of the other end of the electrode covered by the second wiring, and is characterized in that external light emission efficiency is improved and the variation of the external light emission efficiency is eliminated.
This invention is characterized in that external light emission efficiency is improved in a surface light-emitting element comprising a light-emitting layer, an electrode provided on the light-emitting side of the light-emitting layer for injecting current into the light-emitting layer, and a wiring connected to the electrode straddling a certain side of the electrode by providing an insulating layer under the electrode in such a manner as to lie under the inside of the remaining sides, other than the certain side, of the electrode.
This invention is characterized in that external light emission efficiency is improved in a surface light-emitting element comprising at least two semiconductor layers and including a light-emitting layer by providing an electrode that makes ohmic contact with the light-emitting-side semiconductor layer, a metallic layer that makes ohmic contact with the electrode and Schottky contact with the light-emitting-side semiconductor layer; the electrode extending to a region not covered with the wiring of the light-emitting-side semiconductor layer, by injecting current into the light-emitting layer from the metallic layer via the electrode.
This invention is characterized by a surface light-emitting element comprising a light-emitting layer, an electrode provided on the light-emitting side of the light-emitting layer for injecting current into the light-emitting layer, and a wiring connected to the electrode in which at least part of the peripheral shape of the electrode is of an irregular planar shape to increase the peripheral length of the electrode.
This invention is a self-canning, light-emitting device using surface light-emitting elements of the aforementioned construction, or more specifically a self-scanning type light-emitting device in which a plurality of light-emitting elements having control electrodes with a threshold voltage or a threshold current for light emitting operation are arranged; the control electrode of each light-emitting element being connected to the control electrode of at least one light-emitting element located in the vicinity thereof via a connecting resistor or an electrically unidirectional electrical element, and a plurality of wirings for applying voltage or current from outside are connected to the electrodes for controlling the light emission of each light-emitting element.